1. Field of the Invention
The present invention relates to a method for operating a hybrid drive of a vehicle having at least one internal combustion engine and at least one first electric machine as drive machines of a first drive train of the vehicle, as well as an associated first transmission, the total torque of the drive machines being adapted when shifting the transmission in order to reduce the load at the transmission and/or in order to modify rotational speeds according to the new transmission ratio; in addition or as an alternative, in a transition between acceleration operation and trailing throttle operation, the torque gradient of the drive machines is restricted to a specifiable, maximum torque gradient of the resulting total torque of the drive machines of the first drive train.
2. Description of Related Art
Such a method for operating a hybrid drive of a vehicle is generally known. In hybrid drives the internal combustion engine, one or a plurality of electric machine(s) and a transmission of the first drive train are frequently combined into one drive unit and jointly mounted inside the vehicle. The drive unit is connected to a vehicle body of the vehicle via soft bearing elements in order to keep the vibrations produced by the internal combustion engine as well as structure-borne noise away from the vehicle body.
Rapid load changes can generate undesired jerking of the vehicle, in which the moment of inertia of the drive unit typically vibrates against the reduced vehicle mass. Still other forms of vibrations are possible as well. Known methods for reducing bucking vibrations are based on avoiding the excitation of the first drive train by rapid load changes. To this end, in rapid torque changes or high torque gradients, the torque requested by the driver via the driving pedal is filtered, using a low-pass filter, with the aid of a reference-forming unit, and/or its torque gradient is restricted to a maximum torque gradient. This causes a delay in the torque generation or the torque reduction. A zero crossing of the resulting total torque of the drive machines of the first drive train during the transition from trailing throttle to acceleration operation, for example, leads to a zero crossing of the reaction torque and to tilting of the drive unit in the soft bearing elements, which induces vibrations of the first drive train. In addition, mechanical plays or slacks existing in the drive train are run through. For comfort-related reasons, this transition should be a “smooth” transition, which is achieved by restricting the gradient of the resulting total torque of the drive machines of the first drive train during its zero crossing. This, too, is the task of the reference-forming element. It coordinates the drive machines, especially the drive torques of the drive machines (internal combustion engine and first electric machine) of the first drive train. In the case of a hybrid drive designed as parallel hybrid drive and equipped with a crankshaft starter generator as electric machine, a flywheel of the internal combustion engine in particular being coupled to the electric machine, the zero crossing of the summed torque of internal combustion engine and electric machine causes the drive unit to tilt.
In a shifting operation of the transmission of the first drive train, drive torques must be adapted to the requirements in force during the shifting operation as quickly as possible in order to provide rapid, comfortable shifting operations. To this end, the drive machines of the first drive train are controlled in a coordinated manner. The total torque of the drive machines is adapted when shifting the transmission in order to reduce the load at the transmission and/or in order to modify rotational speeds—in particular of inert masses—according to the new transmission ratio. In the case of the parallel hybrid having a crankshaft starter generator as electric machine, the summed torque of internal combustion engine and electric machine is acting at the input of the first transmission of the first drive train, for example. If this transmission cannot be shifted under load, then the summed torque must be brought to zero during the shifting operation. Depending on the operating state, this may be accomplished by lowering the drive torque of the electric machine, or else also by increasing it, e.g., when the summed torque is negative prior to the shifting operation.